fix(billing): keep spacing and link styling consistent when permissions are denied (#12801)

#### Description

- The denied usage callout on the billing page keeps the same 8px gap to
the blocks below it that the usage table has, so the layout reads the
same with or without `subscription:read`.
- "pay the bill" in the failed-payment banner is an inline link again,
with a visible disabled state when the user lacks permission.

#### Additional Information

Part of SigNoz/platform-pod#3091.